1. Field of the Invention
The present invention relates to a bilateral communication system by a optical wireless communication, particularly relates to a optical wireless communication system where information frames are exchanged with an optical node mounted on a mobile object via a plurality of optical repeaters arranged in a wired network system.
2. Brief Description of the Related Art
Recently a wireless optical network (LAN) utilizing infrared rays was proposed and it has put to a practical use. FIG. 6 shows an outline of a system arrangement of such wireless optical networks utilizing infrared rays.
In the system illustrated in FIG. 6, a plurality of optical repeaters AP functioning as access points, are arranged on a ceiling or walls of a building. These optical repeaters AP are connected to a trunk network (not shown in FIG. 6) via a transit post 1, sometimes called “a switching hub”, and optical repeaters AP optically and mutually communicate with optical nodes RN arranged in the building. Each node is connected to a computer PC used as an information processor such as a personal computer or the like.
The above-mentioned optical repeaters AP and optical nodes RN respectively have photodetectors and photo-emitting devices so that the optical repeaters AP and optical nodes RN can mutually communicate. Optical repeaters AP can communicate with a plurality of optical nodes RN, but usually they are connected to a wired network system (fixed system) so as to constitute a large scaled communication system. If optical nodes RN are utilized as mobile units, a quite flexible communication service can be provided.
One optical node RN is usually connected to one computer PC via an interface, which usually employs a versatile wire LAN format. Consequently, the computer PC can be transferred quite easily (can attain a mobile circumstance) without losing same communicating conditions as those of the wired network. Sometimes a plurality of computers PC are connected to one optical node RN. When the interface for optical communication is built in the computer PC, a more simple communicating system is attained.
FIG. 7 shows photo-detecting/emitting areas (communication areas) of the optical repeater AP and the optical node RN. Hereinafter, a communicating direction from the optical repeater AP to the optical node RN is defined as “a downward direction” and the opposite direction is defined as “an upward direction”.
As shown in FIG. 7, the photo-detecting/emitting areas of the optical repeater AP are usually set wide directive angles for more flexible arrangements. On the other hand, the photo-detecting/emitting areas of the optical node RN are set narrow directive angles so as to raise an emitting distance ratio against an inputted power by raising a photo-detective sensitivity and an emitting power.
It is desirable to set the same directional angles for the photo-detecting area and photo-emitting area (namely, for upward and downward directions). Because once a bilateral LAN is employed, transmitted signals should be received without fail. And because a congestion of information in the system is aggravated when redundant information is received, so that a performance of the LAN is deteriorated. It is also desirable to coincide the two areas from a point of a power efficiency in the LAN system.
Although in the above-mentioned conventional optical wireless communication system, the computer connected to the optical node can be used as a mobile unit, usually the computer is considered a fixed unit to a determined position. When the computer is mounted on a mobile object such as a conveyor, a robot or the like, and is required to communicate while moving, the narrow directional angle of the optical node causes a problem. As measures against the problem, two solutions are probable.
A divided photodiode is employed as a photo-detecting member of the optical node and an automatic servomechanism is attached to the optical repeater so as to face directly to the photo-detecting/emitting front of the optical node.
Both directive photo-detecting/emitting angles of the optical node are set wide.
When solution (1) is employed, very complicated mechanism and controlling system are required for attaining the automatic servomechanism. As a result, volume, weight and required power are increased in the system, which will be controversial factors when this solution is employed in the mobile object. Consequently, this solution would require a much higher cost.
Further if wider photo-detecting/emitting areas are attained by employing a plurality of optical repeaters, the more fatal following problem will be caused. One optical repeater cannot be handed over to other optical repeaters, unless an additional detecting system is required in the optical node for recognizing other optical repeaters except the optical repeater currently communicating with the optical node.
Consequently, solution (2) for setting more wide directive angles of the optical node, is considered to be more effective. An example of the wide directive angles is shown in FIGS. 8A and 8B. Reference characters “AP1” and “AP2” in these figures are optical repeaters and a reference character “RN1” is an optical node connected to a computer PC1, which is mounted on a mobile object 2, constituted as a self walking robot. A reference character “PC2” is a computer equipped in a trunk network.
Since a light ray is diffused when transmitted in a space, energy density of the transmitted ray is attenuated exponentially. Therefore, a plurality of optical repeaters AP1, AP2 . . . are arranged so as to widen their combined directive angles for attaining a wide ranged service area.
FIGS. 8A and 8B are schematically illustrated, but actually the following points should be considered.
Boundaries among communicating areas of respective optical repeaters and optical nodes are not distinctively recognized, but can be defined as areas having an error rate lower than a predetermined error rate. In other words, although there are some areas where communication capability is uncertain, the optical wireless communication system is described as a system having distinctive boundaries for easier understanding.
In the bilateral communication system consisting of an upward transmission and a downward transmission, a communicable area (or distance) is determined by a photo-emitting power of the photo-emitting device and a detecting sensitivity of the photo-detector. Actually a communicable area of the upward transmission area does not coincide with the downward transmission area (see FIGS. 9A and 9B).
Hereinafter communication problems caused by the above-mentioned points (a) and (b) are explained by referring to FIGS. 9A, 9B and TAB. 1. FIGS. 9A and 9B show upward and downward transmitting areas in detail. Transmission statuses in respective zones a to g in FIG. 9B, are explained in TAB. 1. A reference numeral “3” in FIG. 9B is a trunk network.
TABLE 1Usable as areaZoneTransmission StatusFor transmission?aOnly upward to AP1 isNoTransmittable.bUpward to/downward fromYesAPI are transmittable.cUpward to/downward fromYes, but sometimesAP1 and upward to AP2 areFaults will occur.transmittable.dUpward to/downward fromYesAP1 and AP2 are transmissible.eDownward from AP1 andYesUpward to/downward from AP2are transmittable.fUpward to/downward fromYesAP2 are transmittable.gOnly downward from AP2 isNoTransmittable.
As shown in FIGS. 9A and 9B, since photo-detecting/emitting angles of the optical node RN1 are already widened, excellent transmission statuses of the area are attained by combined effects of the optical repeaters AP1, AP2 and the optical node RN1.
In order to attain a wide communication area by utilizing a plurality of the optical repeaters AP1 and AP2, it is necessary to place the optical repeaters AP1 and AP2 apart from each other as far as possible so as to overlap respective areas. But not to place too far apart such that no-transmittable zones between the optical repeaters are caused. Hereinafter the situations where the optical node RN1 is situated in overlapped zones (zones c, d and e in FIG. 9) are explained.
An upward transmission of a emitted light signal from the optical node RN1 is detected by the optical repeater AP1 or AP2, from which the signal is transmitted to the trunk network 3. If signals from the optical repeaters AP1 and AP2 are simultaneously transmitted to the trunk network 3, there are probabilities that a data collision will occur and data will be destroyed. In order to avoid such probabilities, it is effective to transmit signals via a transit post 1 equipped with a memory buffer.
The transit post 1 has functions to receive frames (data) from any directions without destroying the frames and to administer sending source addresses of the frames and final destination addresses of the frames so that the frames can be transmitted only to a port where a receiver exists. Consequently, fatal faults do not occur as far as upward transmissions to the optical repeaters are concerned, even if transmittable areas of the optical repeaters are overlapped.
Frames from the trunk network 3 are transmitted downward to the optical repeater AP1 or AP2 where the receiver exists by the above-mentioned functions of the transit post 1. However when the receiver exists (namely the optical node RN1) in the mobile object 2 and even if frames are transmitted to the repeater AP1 or AP2 where the frames existed before, there is a probability that the optical node RN1 to be functioned as the receiver moves out of the transmittable area.
In addition, there is also a problem that an upward transmittable area and a downward transmittable area do not coincide with each other.
In other words there are zones a and g shown in FIG. 9B where only one way transmission is possible, consequently these zones are not usable as the transmittable area. Since in zones b and f only either the optical repeater AP1 or AP2 is bilaterally transmittable, these zones are usable as the normal transmittable area without causing any problems.
Since in zone d both optical repeaters AP1 and AP2 are bilaterally transmittable, when the transit post 1 selects either the optical repeater AP1 or AP2, transmitted frames from the optical repeater AP1 or AP2 arrive at the trunk network 3 without fail.
Since in zone e transmitted frames arrive at only the optical repeater AP2, the transit post 1 selects the optical repeater AP2 as downward transmission, the transit post 1 can bilaterally communicate with the optical repeater AP2.
Zone c is the most controversial zone. Transmitted (emitted) frames in this zone arrive at both optical repeaters AP1 and AP2. A current position of the mobile object 2 equipped with the optical node RN1 is judged as unidentifiable from administration tables (including address tables) in the transit post 1 due to its administrative function. (Which means the transit post 1 transmits frames downwardly to the optical repeater, which recognizes the optical node most currently).
However, even when frames transmitted (emitted) from the optical repeater AP2 do not arrive at the optical node RN1 at zone c, since zone c is out of the downward transmission area of the optical repeater AP2. Although zone c is situated in an overlapped area of optical repeaters AP1 and AP2, communication faults might occur.
Hereinafter, communication problems in a conventional network system are explained. An example of network communication by the above-mentioned system is illustrated in FIG. 10.
In LAN systems, all data are divided into frame units and communicated. By referring to FIG. 10 a simple communication example, where an inquiry frame Q is transmitted from a computer PC1 to a computer PC2 and then the computer PC2 returns a response frame A to the computer PC1, is considered.
The inquiry frame Q transmitted from the computer PC1 is converted into an optical frame by the optical node RN1 and emitted into space. In this case, a photo-emitting angle of the optical node RN1 is set a wide directional angle so that the optical frame transmitted from the optical node RN1 can be received by both optical repeaters AP1 and AP2. The optical frame received by the optical repeaters AP1 and AP2 is converted into respective electrical signal frames and transmitted to the transit post 1 via a wire. In this stage, electrical signal frames corresponding to the number of the optical repeaters which receive the inquiry frame Q from the node RN1, are generated.
Transmitting timings of electrical signal frames from the optical repeaters are determined respectively by optical repeaters AP1 and AP2, so that sometime the timings are delayed each other, but some other time the timings coincide with each other.
The transit post 1 can receive both electrical signal frames from respective optical repeaters AP1 and AP2. Both received electrical signal frames are transmitted to existing ports in the computer PC2 based on receivers'address information included in the electrical signal frame. Since the transit post 1 cannot transmit two frames to the same port simultaneously, either one of frames is transmitted previously based on received orders and processed timings of the received frames. Usually an address table in the transit post 1 is maintained according to the sender's address of the frame transmitted lastly.
In the example illustrated in FIG. 10, since the transit post 1 lastly transmits arrived electrical signal frame from the optical repeater AP2 to the computer PC2, the address table in the transit post 1 recognized that the computer PC1 is connected to (a port for) the optical repeater AP2.
Then the computer PC2 returns response frames A1 and A2 respectively to inquiry frames Q1 and Q2 from the optical repeaters AP1 and AP2. In this case, if a protocol (for example TCP/IP) for administrating frame number is employed, a protocol stack in the computer PC2 is confused so that transmitting performance is remarkably deteriorated. Even if a protocol not for administrating frame number is employed, transmitting performance is also deteriorated due to a redundant frame is transmitted in the network as illustrated in FIG. 10.
In other words, since the same frames are transmitted from the optical repeaters AP1 and AP2 to the trunk line, transmitting performance is deteriorated.
A functional framework of the transit post (switching hub) 1 is illustrated in FIG. 11. The transit post 1 is slightly different from a switching hub (such as 10BASE-T, 100BASE-T or the like) usually employed in a wired LAN such that the transit post 1 has an address administration table 1a and a frame sorting unit 1b. 
When the Ethernet® 4 connected to the trunk port and optical repeaters AP1 to AP4 illustrated in FIG. 11 are employed in an optical wireless LAN, they should have different functions from the wired LAN. Hereinafter required functions are explained.
In the beginning no data are registered in the address administration table 1a. When any one of ports in the transit post 1 is connected to a network, frames are transmitted from the Ethernet® 4. As mentioned before, data in the Ethernet® 4 are divided into frames having a certain length.
First of all addresses peculiar to receivers and senders are added to frames. If a receiver's address in a frame arriving at the transit post 1, is identified as the address registered in the address administration table 1a, the frame is transmitted only to a port corresponding to the registered address. If the receiver's address is not found in the table 1a, the frame is not transmitted to all ports. As a result, since frames are not transmitted to ports having no corresponding registered addresses, congestion in the network is mitigated so that performance is improved. However, when the receiver's address is broadcast information, frames are transmitted to all ports.
Senders' addresses of all frames passing through the transit post 1 are checked according to the address administration table 1a. If a sender's address is not registered in the table 1a, it is registered in the table together with a corresponding port number. When the sender's address is different from a registered port number, the structure of the network is considered to be changed, and the registered number is deleted. Then a new port number is registered instead. In some cases, time information (time stamp) of the registered time is also memorized. When the registered time is judged to elapse more time than a predetermined time, registered data may be deleted.
In addition, there are special frames called “Multi-cast” and “Broad-cast”. Receivers' addresses are not specified in these special frames. When the transit post 1 receives these special frames, they are transmitted to all ports.
Functions described above are the same ones in the optical wireless LAN and the wired LAN. However, the optical wireless LAN had better have a function to transmit an inputted frame to the same port to which an optical repeater is connected. Because, when a plurality of optical nodes are positioned in the same communication area of one optical repeater, frame transmission among the plurality optical nodes are required.
The wired LAN is constituted such that when one node transmit a frame, all other nodes connected to the LAN receive the transmitted frame. Folded transmissions are not necessary in the wired LAN when a receiver and a sender happens to be the same port. On the other hand in the optical wireless LAN, each optical node receives (recognizes) a light emitted form an optical repeater, but it does not always receives emitted lights from other optical nodes due to mutual geometrical relations among the optical nodes. Accordingly, folded transmissions by the optical repeater are required. If the folded transmissions are transmitted, functions of the wired LAN will be deteriorated (a frame congestion will be caused).
Since it is very likely that optical nodes in the wireless LAN move in complicated manners, a high processing speed is required in order to renew the address administration table 1a. 